Driveline retarders are used to apply braking torque to a vehicle transmission, especially in heavy-duty commercial vehicles when driving on a steep grade. For example, a hydraulic driveline retarder may use a rotary member connected to rotate with a transmission output shaft. The rotary member circulates fluid between vane members on a stationary housing surrounding the output shaft. The fluid circulation results in power absorption and the reduction in rotational speed of the output shaft. Various types of driveline retarders are well known. For example, a hydro-dynamic/hydraulic retarder circulates pressurized fluid within a rotor that is enclosed within a separate, vaned stationary housing in order to induce a viscous drag by way of an opposing fluid coupling effect, thus slowing a rotating drive shaft in proportion to the fluid pressure and/or flow, which is controlled by an electro-hydraulic control system based on vehicle operating conditions and/or operator input or request.
Typically, a single variable bleed solenoid is used to control both: (i) a retarder flow valve that permits pressurized fluid into the retarder cavity; and (ii) pressure in the retarder cavity. A separate variable bleed solenoid is used to modulate the main pressure of hydraulic fluid used to fill the retarder and for other transmission needs, such as the engagement of torque-transmitting clutches that establish a desired speed ratio through the transmission.